1. Field of the Invention
The present invention relates to a system and method for accurately accessing a target one of the tracks formed on an optical disc. The track accessing system and method can smoothly read information from an optical disc.
2. Background of the Art
An access method for accessing a desired track during a playback (or during recording) of an optical disc includes a combination of one track jump, jumping a light spot over one track, a multiple track jump jumping the light spot over 10 to 100 tracks at one time, and a large track jump moving a whole optical head in response to individual jump instructions.
FIG. 6 shows a relationship between an optical disc and tracks during an operation of the multiple track jump.
When a light spot placed on a track Q.sub.1 is jumped toward a track Q.sub.2, a number of tracks N over which the light spot is traversed is calculated. Then, an acceleration pulse having a pulsewidth T.sub.a corresponding the calculated number of tracks N is supplied to a fine tracking actuator (hereinafter, simply referred to as an actuator), as shown in FIG. 7.
Thereafter, when a deceleration pulse having a pulsewidth T.sub.b substantially the same as that of the acceleration pulse is applied to the actuator, a brake is applied to the actuator so as to reduce a relative speed V between the optical disc and light spot to zero. Consequently, the light spot can jump the multiple number of tracks N access to the track Q.sub.2 from the track Q.sub.1.
On the other hand, such an access method as will be described below has been proposed.
That is to say, when the acceleration pulse P.sub.a is applied to the actuator in response to the corresponding jump instruction and the light spot is moved at a predetermined speed V, the number of pulses of traverse signals S.sub.t outputted when the light spot traverses one of the tracks are counted. The counted number reaches N-1 with respect to a desired number N. At this time, a brake pulse P.sub.b is applied to attenuate the speed and a track servo circuit is closed to complete the access operation, as shown in FIG. 8.
However, since, in the above-described multiple track jumping method, after the actuator of an optical head is accelerated, no monitoring is carried out for the relative or traverse speed between the tracks in the optical disc and light spot, both a distance of track jump and the relative speed are not constant under an influence of a spring position of the actuator before the jump of the light spot and vibrations during the jump of the light spot. Therefore, a smooth drawing-in of the corresponding track position is not always carried out.
In addition, since a center of rotation does not coincide precisely with an optical disc track and there is an eccentricity between the center of rotation of the disc and the disc track, the relative speed between the optical disc and the light spot is varied, overpassage through the target track occurs, and the traverse speed becomes zero immediately before the target track. In extreme cases, the drawing of the tracking servo becomes impossible.
Although a speed sensor is mounted in a tracking actuator to control the traverse speed, a cost of manufacturing the access system becomes accordingly increased and responsiveness of the actuator becomes reduced.